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7 Calibration RSSI
7.1
What is RSSI
In the mobile phone, the received RF-signal strength is measured and indicated by a function
called RSSI, Received Signal Strength Indicator.
When switching the mobile phone "ON" it starts searching the surrounding radio-channels
ARFCN at the geographical site.
Receiving a lot of information on the broadcast channels it will be directed by the network to
"Camp on a cell".
The surrounding base-stations BSS are at the same time stored in the MS memory as Base
Transceiver Stations Identity Codes, BSIC for further usage. Up to 30 BSIC can be available,
depending on the network. The MS memory capacity is 32 positions for BSIC.
Once the MS camped on the strongest available carrier it starts performing the RSSI
measurements on the surrounding cells from the list of identified BSS-carriers in its memory.
The measurements are averaging from all the base stations by sampling 5 times on each carrier
and then stored in the memory together with the corresponding BSIC.
This list of identified carriers and signal-strength is updated by continuously repeated
measurements.
Another list of carriers and signal-strength is monitored by the MS for Hand Over purposes
during a call in progress. The serving cell is also measured during the cell.
The measurement results are equally stored in the MS memory.
The MS evaluates the RX levels from the measurements results on all carriers ordered by the
network and makes a "TOP 6"-ranking list. The list is recovered once a measurement result
differs from previous value.
The 6 STRONGEST RSSI levels from the surrounding non-serving carriers are then reported
up-link to the serving cell during a call in progress. This report is repeated within 30 seconds if
an RX level or BSIC has been changed.
The signal strength report is used in an evaluation process for Location and Hand-over, i.e.
when the switch evaluates the speech quality, signal strength and traffic parameters to be outside
the limit values of the current physical channel and chooses to start a new channel for the
connection.
A physical channel is the combination of a timeslot (TS) and a radio channel (ARFCN). The
physical channel (TS/ARFCN) can be allocated to the current base station or any of the
surrounding base stations at hand-over.
For the speech quality and the MS to Base distance it is important that the reported RF-signal
measurements are correct and calibrated towards known values. If the reported value is too high
it results in late hand-over and bad readability due to the limits are set out of reach for the MS.
The opposite, too low values, provokes the switch to make unnecessary hand-over, increased
traffic load and perhaps dropped calls by forced release.
The receiver antenna signal carries information both of the phase as well as the amplitude.
The first one is the digital information about the message and it's detected later in a phase
digitizer for further processing in the main program as speech data and signalling.
4/00021-3/FEA 209 544/18 B
Trouble Shooting Guide, Advanced
31(121)
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